1. Field of the Invention
The present invention relates to a method for the determination of certain physical properties of fluids and more particularly the determination of these properties at given reference conditions of temperature and pressure.
2. Background
Traditionally the determination of these properties of fluids at given reference conditions of temperature and pressure has been achieved by temperature and/or pressure control of the gas or liquid of interest, or by means of composition analysis without such controls, either of these at great cost in hardware and energy. This also makes battery powered operation unattractive.
In the copending application Ser. No. 210,892, filed Jun. 24, 1988, entitled "Measurement of Thermal Conductivity and Specific Heat", now U.S. Pat. No. 4,944,035, assigned to the same assignee as the present invention and incorporated herein by reference to the extent needed, there is described that in the prior art determination of specific heat, c.sub.p, has been via calorimetry using reversible step increases of energy fed to a thermally isolated or adiabatic system. Such devices are bulky, slow and cumbersome.
With respect to measuring thermal conductivity in fluids various types of detectors have been used. This includes resistance bridge type sensors. One such device is described in U.S. Pat. No. 4,735,082 in which thermal conductivity is detected using a Wheatstone bridge technique in which a filament in one diagonal of the bridge is placed or positioned in a cavity through which the sample gas of interest is passed. The filament is used to introduce a series of amounts of thermal energy into the fluid of interest at alternating levels by varying the input voltage which, are, in turn, detected at the other diagonal as voltage difference signals. Integration of the changes of the value of the successive stream of signals yields a signal indicative of the heat dissipation through the fluid, and thus, the thermal conductivity of the fluid.
Further to the measurement of thermally induced changes in electrical resistance, as will be discussed in greater detail below, especially with reference to prior art FIGS. 1-5, recently very small and very accurate "microbridge" semiconductor chip sensors have been described in which etched semiconductor "microbridges" are used as condition or flow sensors. Such sensors might include, for example, a pair of thin film sensors around a thin film heater. Semiconductor chip sensors of the class described are treated in a more detailed manner in one or more of patents such as U.S. Pat. Nos. 4,478,076, 4,478,077, 4,501,144, 4,651,564 and 4,683,159, all of common assignee with the present invention.
It is apparent, however, that it has been necessary in the past to address the measurement of specific heat c.sub.p, and thermal conductance, k, of a fluid of interest with separate and distinct devices. Not only is this quite expensive, it also has other drawbacks. For example, the necessity of separate instruments to determine specific heat and thermal conductivity may not allow the data consistency and accuracy needed for useful fluid process stream (gas or liquid) characterization because the required degree of correlation may not be present.
The copending application referred to above addresses an invention which overcomes many disadvantages associated with the determination of both specific heat, c.sub.p, and thermal conductivity, k, by providing simple techniques which allow accurate determination of both properties in a sample of interest using a single sensing system. That invention contemplates generating an energy or temperature pulse in one or more heater elements disposed in and closely coupled to the fluid medium (gas or liquid) of interest. Characteristic values of k and c.sub.p of the fluid of interest then cause corresponding changes in the time variable temperature response of the heater to the pulse. Under relatively static sample flow conditions this, in turn, induces corresponding changes in the time-variable response of one or more temperature responsive sensor coupled to the heater principally via the fluid medium of interest.
The thermal pulse of a source need be only of sufficient duration that the heater achieves a substantially steady-state temperature for a short time. This pulse produces both steady-state and transient conditions at the sensor. Thermal conductivity, k, and specific heat, c.sub.p, can be sensed within the same sensed thermal pulse by using the steady-state temperature plateau to determine k which is then used with the rate of change of temperature in the transient condition to determine c.sub.p.